Insulated gate semiconductor devices such as, for example, Insulated Gate Bipolar Transistors (IGBTs) or Metal Oxide Semiconductor Field-Effect Transistors (MOSFETs) are widely used as electronic switches in various types of electronic circuits in automotive, industrial, consumer electronics, or household applications, to name only a few. An IGBT is a bipolar semiconductor device that includes a first emitter region (also referred to as source region) of a first conductivity type (doping type), a second emitter region (also referred to as drain region) of a second conductivity type, a base region (often referred to as drift region) of the first conductivity type, a body region of the second conductivity type between the first emitter and the base region, and a gate electrode adjacent the body region and dielectrically insulated from the body region by a gate dielectric.
An IGBT can be operated in two different operation states, namely a conducting state (on-state), and a blocking state (off-state). In the conducting state, the first emitter region injects charge carriers of the first conductivity type through a conducting channel in the body region into the base region, and the second emitter region injects charge carriers of the second conductivity type into the base region. These charge carriers injected into the base region by the first and second emitters form a charge carrier plasma in the base region. In the blocking state the conducting channel in the body region is interrupted.
When the IGBT turns off, that is, switches from the conducting state to the blocking state a depletion region expands into the base region beginning at a pn junction between the body region and the base region. Through this, charge carriers forming the charge carrier plasma are removed from the base region. During turn-off there is a current flowing between the first and second emitter region resulting from the removal (extraction) of charge carriers from the base region. This current, which may be referred to as charge carrier extraction current, finally drops to zero as the charge carriers have been removed or recombined. A slope of this current as it tends to zero defines the softness of the component. The steeper the slope, the less “soft” is the turn-off behavior (switching behavior) of the semiconductor device. However, a soft switching behavior is desirable, because steep slopes may cause voltage overshoots in (parasitic) inductances connected to the semiconductor device and/or may cause oscillations or ringing in a circuit in which the semiconductor device is employed.
There is therefore a need to provide an insulated gate semiconductor device such as an IGBT with a soft switching behavior.